The immune response to polysaccharide (PS) antigens is highly regulated and has several distinguishing features including restricted subclass, variable region gene usage, fine specificity, and avidity. Simple PS not conjugated to protein (such as Neisseria meningitidis group C (MCPS)) elicit a thymus-independent (TI) response. PS conjugated to proteins (such as MCPS coupled to tetanus toxoid, (MCPS-TT)), on the other hand, elicit a different type of response, termed thymus-dependent (TD). Previous analyses of anti-MCPS mAb reveal that VH gene family usage is dominated by VHJ558. Antibody affinity may be an important determinant of host defense and should be considered as important as concentration in evaluating antibody response to vaccination. Therefore the IgA, IgG and IgM mAbs from the 3 panels were purified and tested for avidity by a quantitative measure of binding to purified PS in a fluorescent ELISA (FELISA) assay. Both panels of mAbs (C2 and CP), in general, had 2 orders of magnitude lower concentrations for 50% binding, indicating that the TD mAb were of significantly higher avidity (10-100 fold higher) than the anti-MCPS. The ability of the mAb to precipitate PS in gel were also studied and correlated well with the specificity determined by quantitative FELISA. Sequence analysis in progress will determine if somatic mutation can account for the increased avidity . Earlier mouse model studies in our laboratory indicated a developmental delay in the immune response to MCPS even when it was administered as TD MCPS-TT conjugates. Therefore, studies were undertaken to examine whether the delay in response TD conjugates in neonatal mice is due to defective antigen presentation. As a first step, TT and MCPS-TT reactive helper T cell clones were generated from mice that were immunized with MCPS-TT. The T cells clones namely P1/7, P4/19, P4/20 and P4/40 were used to identify the antigen presenting cells involved in the presentation of TT or MCPS-TT. The results showed that adult B cells were much more effective in presenting TT or MCPS-TT to T cells than total spleen cells or macrophages. The ability of B cells to present TT or MCPS-TT to the T cell clones were 4-6 fold higher than total splenocytes. Adult B cells were also found to be several fold more efficient than enriched macrophages in presenting these antigen. However, adult dendritic cells were found to be most effective antigen presenting cells in this system and the efficiency was about 2-fold higher than B cells. Further studies will examine the role of antigen-specific B cells in this process.